Unable to update my iOS

Have no idea what iPhone you have or the iOS version it is using. That would be important to know if you are even eligible for an update. Check at Settings > General > Software Update and see what it says.

Update iOS on iPhone - Apple Support

Beyond that there are a couple of reasons why you would not be able to update, even if you are eligible.

• You may have a VPN installed that can fail to verify the update.
• You may not have enough storage to download and install the update.

Let's first find out if your iPhone is qualified for an update.

Tap Settings > General > About

What iOS Version is your iPhone using?

Tap Settings > General > About

What Model Name is your iPhone?

Tap Settings > General > Software Update

What is the exact message that appears there?

No idea, since you are not providing any information. What model phone is it? What version of iOS is currently on it? What happens when you try to update?